The two types of audio data are very different. I
am concentrating on sampled audio data at this point in time. I will defer
my discussion of
MIDI until later.

Viewing tip

You may find it useful to open another copy of this lesson in a separate
browser window. That will make it easier for you to scroll back and
forth among the different listings and figures while you are reading about
them.

Supplementary material

Related Articles

I recommend that you also study the other lessons in my extensive collection
of online Java tutorials. You will find those lessons published at
Gamelan.com.
However, as of the date of this writing, Gamelan doesn't maintain a consolidated
index of my Java tutorial lessons, and sometimes they are difficult to
locate there. You will find a consolidated index at www.DickBaldwin.com.

The previous lesson showed you how to use the Java Sound API to write
programs to capture microphone data into audio files types of your own choosing.
At that time, I told you that you should be able to play the audio files back
using readily available media players, such as the Windows Media Player.

In
this lesson, I will provide and explain a program that you can use to play back
audio files, including those that you create using a Java program, as well as
those that you acquire from other sources.

Discussion
and Sample Code

What is a SourceDataLine?

I will be using a SourceDataLine object in this program. A
SourceDataLine object is a streaming mixer input object. (A previous
lesson explained lines and mixers in detail.)

A SourceDataLine object can accept a stream of audio data and push that audio data into
a mixer
in real time. The actual audio data can derive from a variety of sources, such
as an audio file, (which will be the case in this program) a network
connection, or a buffer in memory.

Confusing terminology

The terminology used with the Java sound API can be very confusing. Here
is part of what Sun has to say about a SourceDataLine object.

"A source data line is a data line to which data may be written. It acts as a
source to its mixer. An application writes audio bytes to a source data line,
which handles the buffering of the bytes and delivers them to the mixer. The
mixer may ... deliver the mix to a target such as an output port (which may
represent an audio output device on a sound card).

Note that the naming convention for this interface reflects the
relationship between the line and its mixer. From the perspective of an
application, a source data line may act as a target for audio data."

The data written to the SourceDataLine object can be pushed into a mixer in real time.
The
actual destination of the audio data can be any of a variety of
destinations.

(The sample program in this lesson writes audio data into a SourceDataLine
object, which pushes the data to the computer speakers.)

The user interface

When this program is executed, the GUI shown in Figure 1 appears on the screen.
As you can see, this GUI contains the following components:

A text field

A Play button

A Stop button

Figure 1 Program GUI

Operation of the program

The user enters the path and file name for an audio file into the
text field, and then clicks the Play button. The contents of the
audio file are played back through the system speakers.

Playback continues until the system encounters the end of the audio file, or the
user clicks the Stop button, whichever occurs first.

Although the user can terminate the playback of the audio file by clicking the
Stop button, because the audio data is buffered in a large buffer in the playback loop, there may be a
noticeable delay between the time that the Stop button is clicked and the time that the playback actually terminates.

Audio data format is displayed

The program also displays the format of the audio data before playing the file.
The format is displayed on the command- line screen.

Will discuss in fragments

As usual, I will
discuss this program in fragments. A complete listing of the program is
shown in Listing 17 near the end of the lesson.

The class named AudioPlayer02

The program named AudioPlayer02 demonstrates the use of a Java program to
play back the contents of an audio file. The class definition for the controlling class begins in Listing 1.

Several instance variables are declared in Listing 1. Some of those
instance variables are also initialized. It is worth noting that the text
field object is initialized to contain the default file name junk.au.
This makes it possible to run the program without the requirement to type the
file name in the text field, provided that an audio file named junk.au
resides in the same directory as the controlling class file. (A
previous lesson contained a program that created an audio file named junk.au.)

The main method

The main method for this Java application, shown in Listing 2, is
extremely simple.

The code in the main method simply instantiates an object of the
controlling class. Code in the constructor, some other methods, and some
inner classes take over at that point and provide the operational behavior of
the program.

I explained this anonymous inner class syntax in the previous lesson. The
most significant thing about the code in Listing 4 is the statement highlighted
in boldface. This statement invokes the playAudio method to play
the contents of the audio file when the user clicks the Play button.

The other two statements in Listing 4 simply reverse the enabled/disabled state
of the Play and Stop buttons.

Action listener on the Stop button

We are still discussing the constructor for the controlling class. The
code in Listing 5 instantiates and registers an action listener on the Stop
button.

When the user clicks the Stop button, the code in Listing 5 sets a flag
named stopPlayback to true. As you will see later, the thread that
handles the actual playback operation periodically tests this flag. If the
flag switches from false to true, the code in the run method of the
playback thread terminates playback at that point in time.

There may be a time delay before termination of
playback

However, there are some rather large buffers used in the playback loop.
These buffers must be emptied before playback actually terminates. Therefore,
there may be a time delay between the point in time that this flag is set to
true and the point in time that playback actually terminates.

Places the text field and the two buttons in their respective positions in
the GUI.

Sets the title and the size of the GUI.

Enables the X button in the upper-right corner of the frame to terminate
the program when it is clicked.

Makes the whole thing visible.

The playAudio method

Recall that the event handler on the Play button invokes the playAudio
method. The playAudio method plays back audio data from an audio file,
where the name of the audio file is specified in the text field of the GUI. The
playAudio method begins
in Listing 7.

Listing 8 passes the File object's reference to the static
getAudioInputStream method of the AudioSystem class. There are
several overloaded versions of the getAudioInputStream method in Java SDK
version 1.4.1. Here is part of what Sun has to say about the version of
the method used in Listing 8.

"Obtains an audio input stream from the provided File. The File must point to
valid audio file data."

The audio format

You learned in previous lessons that audio data comes in a variety of audio
formats. The first statement in Listing 9 invokes the getFormat method on
the AudioInputStream object to get and save the audio format of the specified
audio file.

Here is the reported format for a file named tada.wav, which I found in
the operating system media directory on my Win2000 system.

PCM_SIGNED, 22050.0 Hz, 16 bit, stereo,
little-endian, audio data

Here is the reported format for a file named junk.au, which was produced
by the program named AudioRecorder02 in the previous lesson.

PCM_SIGNED, 8000.0 Hz, 16 bit, mono,
big-endian, audio data

Be careful with the backslashes

If you
are using a Windows system, you must either specify the path and file name in
the GUI text field using forward slashes, or by using double backslashes. If
the audio file is in the same directory as the compiled version of this program,
you don't have to specify a path.

A DataLine.Info object

In the previous lesson, you learned of the requirement to create a
DataLine.Info object that described the TargetDataLine that was
needed to handle the acquisition of the audio data from the microphone.

A similar requirement exists in this program. However, in this program, we
need a DataLine.Info object that describes the SourceDataLine
object that we will use to feed the audio data to the speakers. The
DataLine.Info object is instantiated in Listing 10.

Note that in this case, the first parameter to the DataLine.Info
constructor is a Class object that represents the type of an object
instantiated from a class that implements the SourceDataLine interface.

The second parameter to the constructor specifies the format of the audio data
to be handled by the SourceDataLine object.

Getting a SourceDataLine object

Finally, the code in Listing 11 invokes the static getLine method of the
AudioSystem class to get a SourceDataLine object that matches the
description of the object provided by the DataLine.Info object.

I discussed the static getLine method of the AudioSystem class in
detail in the previous lesson, so I won't discuss it further here.

Spawn and start a thread to handle the playback

The boldface statement in Listing 12 creates a thread (to play back the data) and starts
the thread running. The thread will run until the end of the file is
encountered, or the Stop button is clicked by the user, whichever occurs first.
(Because of the data buffers involved, there will normally be a delay between the click on the
Stop button and the actual termination of playback.)

If the read method returns a positive value, that value indicates the
number of bytes that were read from the audio file into the temporary buffer
named tempBuffer.

If the number of bytes that were read is greater than zero, the write
method of the SourceDataLine object is invoked to transfer those bytes to
an internal buffer of the SourceDataLine object.

Convert sampled audio data to sound pressure waves

The SourceDataLine object causes the sampled audio data values to be converted
to analog voltages, which are delivered to the speakers where the voltages are
converted to sound pressure waves.

In order to accomplish this, the SourceDataLine object must know about
the format of the audio data, including the sample rate, the number of channels,
the number of bits per sample, etc. That information was provided to the
SourceDataLine object by way of the DataLine.Info object in
Listing 10.

Drain the SourceDataLine buffer

When the while loop terminates for one of the reasons discussed earlier,
there may still be data in the internal buffer of the SourceDataLine
object. The code in Listing 15 invokes the drain method on the SourceDataLine
object to continue the data transfer in real time until this data is exhausted.

sourceDataLine.drain();
sourceDataLine.close();
Listing 15

Here is part of what Sun has to say about the drain method.

"Drains queued data from the line by continuing data I/O until the data
line's internal buffer has been emptied. This method blocks until the draining
is complete."

The close method

The code in Listing 15 also invokes the close method on the
SourceDataLine object. Here is part of what Sun has to say about the
close method.

"Closes the line, indicating that any system resources in use by the line can
be released."

Thus, when the code in Listing 15 has finished executing, all of the data
delivered to the SourceDataLine object has been passed along to the
speakers, and the SourceDataLine object has release system resources.

Prepare to play back another file

The three statements in Listing 16 reset some values to prepare the program to
play back another file.

You can view the catch block in Listing 17
near the end of the lesson.

Run the Program

At this point, you may find it useful to compile and run the program shown
in Listing 17 near the end of the lesson.

Specify the audio file

Start the program and enter the path and file name of an audio file in the
text field that appears on the GUI shown in Figure 1. You should
separate the directories in the path using forward slashes instead of
backslashes. If you are running under Windows and you want to use
backslashes, use two backslashes in each case in place of one backslash.
If the audio file is in the same directory as the program (the current
directory), you don't need to enter the path.

Start the playback

Click the Play button after entering the file name. Playback
should begin, and should continue to the end of the audio file unless you
click the Stop button while the file is being played.

The Stop button

If you click the Stop button while the file is being played,
playback should terminate. However, because some relatively large
buffers are used in the playback loop, there may be a noticeable delay
between the time that you click the Stop button and the time that
the playback actually terminates.

Volume control

If you don't hear anything during playback, you may need to increase your speaker volume.
My laptop computer has a manual volume control in addition to the software volume
controls that are accessible via the speaker icon in the system tray.

In
this lesson, I have explained a program that you can use to play back
audio files, including those that you create using a Java program, as well as
those that you acquire from other sources.

Complete Program Listing

A complete listing of the program is shown in Listing 17.

/*File AudioPlayer02.java
Copyright 2003 Richard G. Baldwin
Demonstrates playback of an audio file. The
path and name of the audio file is specified by
the user in a text field.
A GUI appears on the screen containing
the following components:
Text field for the file name
Play
Stop
After entering an audio file name in the text
field, the user can click the Play button to
cause the program to play the audio file. By
default, the program will play the entire file
and then get ready to play another file, or to
play the same file again.
If the user clicks the Stop button while the file
is being played, playback will terminate.
However, because the audio data is buffered in a
large buffer in the playback loop, there may be a
noticeable delay between the time that the Stop
button is clicked and the time that the playback
actually terminates.
The text field contains the default audio file
name, junk.au, when the GUI first appears on the
screen.
The program displays the format of the audio
data in the file before playing the file. The
format is displayed on the command- line screen.
Tested using SDK 1.4.1 under Win2000
************************************************/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import javax.sound.sampled.*;
public class AudioPlayer02 extends JFrame{
AudioFormat audioFormat;
AudioInputStream audioInputStream;
SourceDataLine sourceDataLine;
boolean stopPlayback = false;
final JButton stopBtn = new JButton("Stop");
final JButton playBtn = new JButton("Play");
final JTextField textField =
new JTextField("junk.au");
public static void main(String args[]){
new AudioPlayer02();
}//end main
//-------------------------------------------//
public AudioPlayer02(){//constructor
stopBtn.setEnabled(false);
playBtn.setEnabled(true);
//Instantiate and register action listeners
// on the Play and Stop buttons.
playBtn.addActionListener(
new ActionListener(){
public void actionPerformed(
ActionEvent e){
stopBtn.setEnabled(true);
playBtn.setEnabled(false);
playAudio();//Play the file
}//end actionPerformed
}//end ActionListener
);//end addActionListener()
stopBtn.addActionListener(
new ActionListener(){
public void actionPerformed(
ActionEvent e){
//Terminate playback before EOF
stopPlayback = true;
}//end actionPerformed
}//end ActionListener
);//end addActionListener()
getContentPane().add(playBtn,"West");
getContentPane().add(stopBtn,"East");
getContentPane().add(textField,"North");
setTitle("Copyright 2003, R.G.Baldwin");
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(250,70);
setVisible(true);
}//end constructor
//-------------------------------------------//
//This method plays back audio data from an
// audio file whose name is specified in the
// text field.
private void playAudio() {
try{
File soundFile =
new File(textField.getText());
audioInputStream = AudioSystem.
getAudioInputStream(soundFile);
audioFormat = audioInputStream.getFormat();
System.out.println(audioFormat);
DataLine.Info dataLineInfo =
new DataLine.Info(
SourceDataLine.class,
audioFormat);
sourceDataLine =
(SourceDataLine)AudioSystem.getLine(
dataLineInfo);
//Create a thread to play back the data and
// start it running. It will run until the
// end of file, or the Stop button is
// clicked, whichever occurs first.
// Because of the data buffers involved,
// there will normally be a delay between
// the click on the Stop button and the
// actual termination of playback.
new PlayThread().start();
}catch (Exception e) {
e.printStackTrace();
System.exit(0);
}//end catch
}//end playAudio
//=============================================//
//Inner class to play back the data from the
// audio file.
class PlayThread extends Thread{
byte tempBuffer[] = new byte[10000];
public void run(){
try{
sourceDataLine.open(audioFormat);
sourceDataLine.start();
int cnt;
//Keep looping until the input read method
// returns -1 for empty stream or the
// user clicks the Stop button causing
// stopPlayback to switch from false to
// true.
while((cnt = audioInputStream.read(
tempBuffer,0,tempBuffer.length)) != -1
&& stopPlayback == false){
if(cnt > 0){
//Write data to the internal buffer of
// the data line where it will be
// delivered to the speaker.
sourceDataLine.write(
tempBuffer, 0, cnt);
}//end if
}//end while
//Block and wait for internal buffer of the
// data line to empty.
sourceDataLine.drain();
sourceDataLine.close();
//Prepare to playback another file
stopBtn.setEnabled(false);
playBtn.setEnabled(true);
stopPlayback = false;
}catch (Exception e) {
e.printStackTrace();
System.exit(0);
}//end catch
}//end run
}//end inner class PlayThread
//===================================//
}//end outer class AudioPlayer02.java
Listing 17

Copyright 2003, Richard G. Baldwin. Reproduction in whole or in
part in any form or medium without express written permission from Richard
Baldwin is prohibited.

About the author

Richard Baldwin is a college professor (at Austin Community College in Austin, TX) and private consultant whose primary focus is a combination of Java, C#, and XML. In addition to the many platform and/or language independent benefits of Java and C# applications, he believes that a combination of Java, C#, and XML will become the primary driving force in the delivery of structured information on the Web.

Richard has participated in numerous consulting projects and he
frequently provides onsite training at the high-tech companies located
in and around Austin, Texas. He is the author of Baldwin's Programming Tutorials,
which has gained a worldwide following among experienced and aspiring programmers.
He has also published articles in JavaPro magazine.

Richard holds an MSEE degree from Southern Methodist University and
has many years of experience in the application of computer technology
to real-world problems.